The present invention relates generally to earth leakage (ground fault) detection devices. More specifically, the present invention relates to earth leakage detection devices for use with molded case circuit breakers.
An earth leakage detection device is generally installed in an electrical power distribution circuit in conjunction with a molded case circuit breaker. The earth leakage detection device detects the existence of certain predefined earth leakage current levels. If such current levels exist, the earth leakage detection device causes the circuit breaker to trip, thus stopping current flow to the protected circuit. Together, the earth leakage detection device and the molded case circuit breaker provide overcurrent and earth leakage protection to the distribution circuit.
A conventional earth leakage detection device generally comprises a housing in which different mechanical, electrical and electronic elements are enclosed. This housing can be separate from, or integral to, the housing for the associated molded case circuit breaker. Within the housing, the earth leakage detection device includes a plurality of conductive straps, one strap being provided for each pole of the electrical distribution circuit. Each of these straps passes through a torous-shaped, ferrous core mounted within the housing. Typically, the toroidal core and the straps are wrapped in insulative tape. The straps passing through the toroidal core form the primary winding of a current transformer. A secondary winding of the current transformer is electrically connected to earth leakage detection electronics mounted within the housing.
Typically, the principle applied to determine the existence of earth leakage consists of measuring the sum of the electric currents flowing simultaneously in the straps (i.e. each pole of the distribution circuit). When the distribution circuit down-line of the earth leakage detection device functions normally, the sum of the electric current that flows simultaneously though the straps is essentially equal to zero. If there is earth leakage down-line, the sum of the electric currents that flow simultaneously through the straps will no longer be equal to zero and an electric current will be induced in the secondary winding of the transformer. The current induced in the secondary winding is sensed by the earth leakage detection circuitry, which determines the level of current leakage to earth. If detected current level is greater than a predetermined current threshold setting, the earth leakage detection circuitry will provide a trip signal to an electromechanical trip/reset mechanism located within the earth leakage detection device housing. In response to the trip signal, the trip/reset mechanism will trip an operating mechanism within the molded case circuit breaker to stop current flow in the protected circuit. Typically, the predetermined current threshold level and the predetermined trip time can be adjusted using sensitivity adjustment knobs, which extend through the top of the housing of the earth leakage detection device. Current threshold level and maximum trip times are predefined by standards (e.g., Appendix B of IEC 947-2).
In earth leakage detection devices of the prior art, the trip/reset mechanism is rigidly mounted to the support structure for the current transformer. Unfortunately, this arrangement makes the trip/reset mechanism susceptible to the vibration of the current transformer. If the vibration caused by the current transformer (or any other source) is sufficient, the trip/reset mechanism could trip spuriously.
Dielectric testing is performed on the differential circuit breaker to insure adequacy of its insulation. Dielectric testing requires that the technician impart a higher than normal voltage across both the earth leakage detection device and the molded case circuit breaker. Unfortunately, this increased voltage can harm the electronics in the earth leakage detection device. To avoid this damage, the technician must remove the earth leakage detection device from the line before performing this test. However, the removal of the earth leakage detection device is a time consuming process that increases maintenance costs and subjects the earth leakage detection components to damage while they are removed.
In an exemplary embodiment, an earth leakage detection device detects earth leakage in an electrical distribution circuit and actuates a circuit breaker when earth leakage is detected. The earth leakage detection device includes a housing and an earth leakage detection circuit mounted within the housing for detecting earth leakage in the electrical distribution circuit. An electrically conductive strap is arranged to conduct electrical current to the electrical distribution circuit. The electrically conductive strap provides operating current to the earth leakage detection circuit. A dielectric test switch is arranged between said electrically conductive strap and the earth leakage detection circuit. The dielectric test switch includes a button disposed in the housing. When the button is pressed, the dielectric test switch stops the flow of electrical current from the electrically conductive strap to the earth leakage detection circuit to protect the earth leakage detection circuit during dielectric testing. In addition, when the button is pressed, the circuit breaker is actuated.